Interaction of hypoxia and vascular occlusion on cardiorespiratory responses during exercise

Background: The interaction of group III/IV afferents and chemoreflex during exercise is critical in healthy adults during high altitude exercise or in clinical populations who experience hypoxia. Purpose: Investigate the cardiorespiratory response to simultaneous vascular occlusion (to activate group III/IV afferents) and hypoxia (to activate chemoreflex) during cycling exercise. Methods: 18 adults (9 women, 25 ± 5 years) attended two sessions. Session 1: maximal cycle ergometry test. Session 2: two 26-minute bouts (randomized between hypoxia, 12.5% FiO₂ and normoxia, 21% FiO₂). Participants cycled at 30% of peak workload for 3 minutes (control, CTL) followed by alternating 2 minute periods of bilateral vascular occlusion of the proximal thigh at pressures of 20, 40, 60, 80, 100 mm Hg in a randomized sequence. Results: Ventilation (V_E) increased from CTL to 100 mm Hg during hypoxia (39 ± 9 to 51 ± 16 L/min) and normoxia (31 ± 7 to 39 ± 9 L/min, P < 0.01). Respiratory rate increased with vascular occlusion (P < 0.05) but not hypoxia (P = 0.10). Tidal volume was greater during hypoxia (P < 0.05), with no influence of vascular occlusion (P = 0.40). Mean arterial pressure and heart rate increased more with hypoxia compared with normoxia (P < 0.05). Conclusions: Our findings suggest that vascular occlusion and hypoxia both increase V_E, albeit via different mechanisms. While hypoxia increased tidal volume, vascular occlusion increased respiratory rate.